As the semiconductor devices keeps scaling down in size, three-dimensional multi-gate structures, such as the fin-type field effect transistors (FinFETs), have been developed to replace planar Complementary Metal Oxide Semiconductor (CMOS) devices. A structural feature of the FinFET is the silicon-based fin that extends upright from the surface of the substrate, and the gate wrapping around the conducting channel that is formed by the fin further provides a better electrical control over the channel.
Currently, epitaxial source and drain for FinFETs are formed after fin recess process. The epitaxial strained source and drain are required to be embedded in gate stack to enhance device boost of FinFETs. During formation of recesses formed for growth of epitaxial source and drain, the lateral dimension and the depth of the recesses increase simultaneously. However, sub-fin leakage path may occur when the depth of the recesses increase.